FIG. 7 is a block diagram of a velocity control system in a conventional motor control apparatus 200. The motor control apparatus 200 controls a motor 1. A load 2 is connected to a drive shaft 3 of the motor 1. An encoder 4 that detects a position of the motor 1 and that outputs the detected motor position is attached to the motor 1.
The motor control apparatus 200 includes a velocity detecting unit 5, a comparator 6, a velocity control unit 7, and a current control unit 8. The velocity detecting unit 5 calculates a differential of the motor position output from the encoder 4, and calculates a velocity (rotational velocity) of the motor 1 from the differential. The comparator 6 calculates a difference between a velocity command signal received from a higher-level controller or a position controller (not shown) with the motor velocity received from the velocity detecting unit 4, and outputs a velocity error indicative of the difference. The velocity control unit 7 receives the velocity error, calculates a current command that is a command to drive the motor 1 (“motor driving command”), and outputs the current command to the current control unit 8. The current control unit 8 controls the current to be supplied to the motor 1 based on the current command received from the velocity control unit 7. Due of the current supply to the motor 1, a predetermined torque is generated in the motor 1 and the motor 1 is driven.
The velocity control unit 7 includes a comparison controller 9, an integration controller 10, and an adder 11. The comparison controller 9 multiplies the input velocity error by a comparison gain KP, and outputs the result of the multiplication. The integration controller 10 multiplies an integrated value of the velocity error by an integration gain KI and outputs the result of the multiplication. The adder 11 adds the results received from the comparison controller 9 and the integration controller 10, and outputs the result of the addition as the current command.
The conventional motor control apparatus 200 provides a control to generate a torque in the motor 1 so that the velocity error, which is the difference between the velocity command signal and the motor velocity, is smaller. As a result, the motor 1 and the load 2 rotate to follow up the velocity command signal transmitted from the higher-level controller or the position controller. The motor velocity changes if a disturbance torque acts on the load 2. However, this velocity change is detected by the encoder 4 and the velocity detecting unit 5, the detected velocity change is fed back to the velocity control unit 7, and the velocity control unit 7 generates the current command to correct the change in the motor velocity due to the disturbance torque. In this manner, even if a disturbance torque acts on the load 2, because a change in the motor velocity is suppressed by a velocity control loop, the motor 1 can be controlled to always follow up the velocity command signal.
A velocity command signal follow-up speed, a velocity command signal follow-up accuracy, and a performance for suppressing an influence of the disturbance torque are improved if the comparison gain KP and the integration gain KI are higher. Owing to this, these gains are normally set as high as possible. However, if these gains are set too high, then the control system can become instable, and vibrations and oscillations can occur. Therefore, there is a trend to set these gains as high as possible and within a range within which stability margins can be secured.
Phase margin and gain margin are known indexes of the stability margin. There is known a technique for adjusting control parameters such as the comparison gain KP and the integration gain KI in such a manner that the phase margin and the gain margin fall within their respective ranges (see for example, Patent Document 1).
However, in the conventional technique, if the stability margins such as the phase margin and the gain margin are insufficient, the comparison gain KP and the integration gain KI are simply set lower. Due to this, the comparison gain KP and the integration gain KI often cannot be set sufficiently high. As a result, the required control performance cannot be obtained.
Furthermore, Patent Document 2 proposes an improvement in a control capability of the motor control apparatus by providing a notch filter in the motor control apparatus. In this technique, a frequency near an oscillating frequency of the control system or a frequency at which a phase delay begins to appear is set as a central frequency. In addition, a notch filter, which has a small attenuation at the central frequency, is inserted into the control system, thereby improving a phase characteristic of the control system. If the phase characteristic is improved, the control gain can be increased while preventing oscillations. The control performance can be, therefore, improved.
In this technique, however, a degree of improving the phase characteristic of the control system depends largely on characteristics of the notch filter. To improve the phase characteristic, it is necessary to appropriately set the central frequency of the notch filter and the attenuation at the central frequency of the notch filter. However, no document, including the Patent Document 2, teaches how to set the central frequency and the attenuation at the central frequency of the notch filter. Therefore, it is difficult to set the characteristics of the notch filter for different control targets. As a result, a characteristics improving effect by providing the notch filter cannot be attained sufficiently.
Patent Document 1: Japanese Patent Application Laid-open No. 2002-116803
Patent Document 2: Japanese Patent Application Laid-open No. H5-76192